This invention relates to an assembly and an associated method for recovering crystalline material from a slurry.
In the production of terephthalic acid, crystals of terephthalic acid are conventionally removed from a slurry via a rotary vacuum filtration unit. The filtration unit includes a cylindrical filter drum rotatably disposed inside a housing. The slurry is fed to the housing to a point radially outward of the filter. Suction applied inside the filter drum draws the liquid components of the slurry through the filter, with the terephthalic acid being deposited in a layer called a "cake" on the outer cylindrical surface of the drum.
As the filter drum rotates, the deposited crystalline layer is transported past a washing station where a washing fluid or liquor is dispensed onto the layer and drawn through the filter into the rotating drum. Subsequently, the purified terephthalic acid rotates to a discharge station where a jet of pressurized gas from inside the rotating cylinder pushes the deposited cake from the outer surface of the filter and into a discharge outlet.
In certain stages of a terephthalic acid production process, this rotary vacuum filtration technique can have the disadvantage of clogging the filter medium, thereby requiring extended downtime for the filter media replacement or repair. One attempt to obviate this disadvantage of the rotary vacuum filtration technique is to convert the filter to a high pressure and temperature installation, thereby increasing the saturation level of the filtrate and increasing average filter operation times. In accordance with this solution, as described in U.S. Pat. No. 5,093,001 to Ueda, the slurry at the input of the filtration unit is subjected to elevated pressure, thereby forcing fluid through the filter cloth and into the filter drum.
This pressurized rotary filtration technique results in higher pressures throughout the system, including the "cake" discharge orifice. However, a problem arises in containing the pressure within the system while allowing continuous throughput and discharge of the cake to lower or atmospheric conditions for downstream processing.
U.S. Pat. No. 5,093,001 discloses the disposition of a pair of valves at the output of a pressurized rotary filtration unit. However, this configuration is not sufficient to solve the pressure control problem and assist in maintaining a high continuous throughput of the rotary filtration unit.